1. Field of the Invention
The present invention relates to an improvement in a steering gear ratio changing apparatus for changing the turning angle of vehicle wheels in accordance with the analog of rotation of a steering wheel.
An apparatus disclosed in, e.g., Published Examined Japanese Patent Application No. 54-34212 (U.S. Pat. No. 3,831,701) is known as a conventional steering gear ratio changing apparatus.
This apparatus is designed such that a gear ratio changing mechanism of a planetary gear system is arranged in the middle portion of a steering shaft, and a steering gear ratio is changed by the operation of the gear ratio changing mechanism. More specifically, this gear ratio changing mechanism is designed such that input and output shafts are respectively coupled to the sun gears of the first and second planetary gear mechanisms whose planetary gear carriers are coupled to each other. In this arrangement, a differential input is supplied from an actuator to the ring gear of the second planetary gear mechanism so as to control the rotating amount of the output shaft in response to the rotating amount of the input shaft.
In the conventional steering gear ratio changing apparatus, however, since the gear ratio changing mechanism of the planetary gear system is used, play of the steering wheel tends to be produced due to the backlash of the gears.
If the changing apparatus is combined with a power steering unit, steering torque-oil pressure characteristics tend to exhibit excessive hysteresis (a great difference appears in the hydraulic pressure generated by the power steering unit between the turning and returning sides of the steering wheel) due to friction between gears upon rotation.
Another problem of the changing apparatus is that its structure is complicated and large in size. In addition, the strength of each gear against an excessive input from the steering wheel must be ensured, and the ring gear must be actuated by the actuator. For these reasons, it is very difficult to reduce the apparatus in size. Hence, a large space is required to mount the apparatus in a vehicle.